This invention relates to explosives for use in mining, both packaged and bulk explosives, which are not cap sensitive and are commonly called blasting agents. In particular, this invention relates to blasting agents containing particulate solid oxidiser materials of selected particle size.
Bulk blasting agents which are well known in the art include ANFO (a mixture of ammonium nitrate prills and fuel oil), blends of ammonium nitrate or ANFO with an emulsion phase, called Heavy ANFO, emulsion explosives (which sometimes contain some solid oxidiser), and water-gel or slurry explosives. Most of these blasting agents contain solid oxidisers, most commonly ammonium nitrate, which is added as a discrete phase or which results from the crystallisation of nitrates from the aqueous phase when a slurry explosive is prepared hot and allowed to cool before firing.
All the above bulk blasting agents are used in packaged form as well, with the composition being suitably modified. Explosives used in smaller diameters require increased sensitivity, and this can be achieved by the addition of chemical sensitisers, such as mono-methylamine nitrate, ammonium perchlorate and others. In order to ensure adequate shelf life, formulators of packaged explosives commonly include glass micro-balloons, instead of chemical gassing, and use waxes instead of oils in emulsion explosives. Packaged explosives nevertheless often contain solid nitrates which are added into the composition or arise from crystallisation during cooling.
The behavior of solid oxidisers in explosives has an important bearing on their detonation characteristics and on their rock breaking performance. It is well known that solid oxidisers of smaller particle size lead to higher detonation velocity of the explosive, because such finely-divided oxidiser is able to react completely, or almost completely in the detonation reaction zone. Solid oxidisers of larger particle size react only partially in the detonation reaction zone, which results in a lower velocity of detonation. Ingredients in an explosive composition which do not react in the detonation reaction zone nevertheless continue to react and release their energy. Depending on the time-scale of such energy release, it may or may not contribute to the useful work in fracturing, dislodging and moving the rock.
Solid oxidisers for use in blasting agents are selected for various reasons. Ammonium nitrate (AN) is an economical, widely available and energetic oxidiser. In porous prill form, AN absorbs fuel oil to form an effective explosive. The particle size of the prills is typically about 2 mm, but the prill structure is an open or porous lattice of micro-crystals. This ensures intimate mixing of fuel and oxidises and leads to effective shock heating and subsequent reaction at many points within the porous prills. In terms of reactivity, therefore, the particle size of porous prills is effectively much less than the prill diameter.
Solid oxidisers in the form of dense prills, granules or crystals are normally used either in the size range of greatest availability, or are used in the most finely-divided form which can be practically achieved. The use of fine solids is to ensure a high velocity of detonation in the explosive. Sometimes solid oxidisers are selected for their effects on rheology or on product density. Fine solids have a drying effect on liquid/solid systems, and this can be disadvantageous, e.g. in packaged explosives.
Solid oxidisers are therefore commonly selected for reasons of availability, cost, to maximise the velocity of detonation, for their effect on the rheology of a composition, etc. Formulators of explosives choose the most finely-divided form of solid oxidiser available, porous ammonium nitrate being a case in point, provided that some other property of the composition is not adversely affected.